IMPACT of BUFFER STRIP PONDS ON AQUATIC ECOLOGY

SOWAP investigated whether a series of pools created within a riparian buffer strip could be used as an “end-of-pipe” solution to reduce nutrient transport to streams from agricultural drains and ditches.

In theory, buffer strip pools have the potential to reduce pollutant levels in field drainage by providing opportunities for sediment to settle and for nutrient uptake by algae and macrophytes. Similar sequences of ponds are now widely used to combat urban runoff and treat some industrial effluents. However they have not been used or tested often on farmland. Recent evidence suggests that the creation of new ponds may also bring considerable biodiversity benefits.

Pond set-up

The study was undertaken at the Allerton Project's research and demonstration farm at Loddington ( Leicestershire , UK ). Two pool series' were created here (Figure 1), both draining water from arable fields. In the first series (Series A), created in 1998, water passes from a broken field drain into a sequence of seven pools separated by swales of boggy vegetated ground. In the second pool sequence (Series B), water from a ditch, fed by field drains, passes through three pools. The largest pool in the B sequence (B3) was created in 1998, the other two in 2004. An additional pool (C1), created in 1998 receives water from both the bottom of the A-series, and from a broken field drain. Pool surface areas across the site range from approximately 3m 2 to 90m 2 . Most pools were seasonal or semi-permanent with only C1 fully permanent.

The ponds were surveyed in June from 2004-2006.

Buffer-strip ponds benefit water quality

Total phosphorus concentrations at the outflow of Series A and Series B pool sequences were respectively 43% and 52% lower than those recorded in the supplying field drain or ditch (Table 1). For total nitrogen the concentration was 31% and 33% lower in the last pool of the sequence than in the ditch or drain feeding them. The drop in nutrient concentration occurred at an earlier stage in the pool sequence for phosphorus than it did for nitrogen.

Table 1. Mean concentrations of total phosphorus (µg P/l) and total nitrogen (mg N/l) in buffer strip pools and their source ditch or drain

Biological data collected from Series A pools supported the water chemistry findings. The topmost pool (A1), which received the field drain, supported significantly (32%) fewer aquatic invertebrate species than the lowest comparable pool (A5) in the sequence (Figure 2).

Figure 2: Mean aquatic invertebrate and macrophyte species richness in similar pools at the top (A1) and bottom (A5) of the series

Scores for the three larger pools in this sequence (A1, A2 and A5) confirm that the “health” of the pools increased “downstream” through the series as water quality improved. Thus, only the topmost pool (A1) fell below the 75% threshold dividing pools of “Good” and “High” status (Figure 3).

Buffer-strip ponds benefit biodiversity

The buffer strip pools supported a total of 36 wetland macrophyte species, with an average of 7-19 species recorded from individual ponds across the three years. All plant species were common at a national level with the exception of the locally uncommon Marsh Dock which occurred in pool C1.

A total of 131 invertebrate species were recorded from the eight pools. The average species richness of individual ponds ranged from 25- 59 species. The small, shallow and well-vegetated characteristics of the pools encouraged strong breeding populations of dragonflies. Ten dragonfly and damselflies species had breeding populations in the pools with five species particularly abundant: Common Darter, Large Red Damselfly, Blue-Tailed Damselfly, Common Blue Damselfly and Azure Damselfly.

A number of uncommon invertebrates were recorded from the pools. This included eight species of Near Threatened or Nationally Scarce water beetles (Agabus labiatus, Berosus signaticollis, Dryops similaris, Haliplus laminatus, Helophorus granularis, Limnebius nitidus, Limnebius papposus and Rhantus suturalis). Five locally uncommon invertebrate species were also recorded.

The larvae or adults of three amphibian species were found in the pools. These were: Common Frog, Common Toad and Smooth Newt.

A surprising result from the study was that even the smallest pools in the series - simple vertical-sided square holes just 1.5m diameter - supported rich invertebrate populations, with up to 43 invertebrate species and three Nationally Scarce species recorded in a single season. Frog tadpoles and newt larvae were also abundant in these pools.

Buffer-strip ponds bring multiple benefits

There are multiple benefits from the use of buffer strip pools to treat farmland run-off in drains and ditches. They:

• Contribute to improved chemical and, potentially, biological status of lakes and watercourses

• Create new freshwater habitats of value in their own right, potentially increasing freshwater biodiversity at local and regional scale. The eight buffer strip pools at Loddington supported over 25% more invertebrate species and 37% more nationally uncommon species than the 15 streams spread across the English Midlands, assessed by SOWAP to compare soil management methods. This result is consistent with other work, showing that ponds can contribute a very high proportion of the total freshwater biodiversity in agricultural catchments, and that new ponds are an important component of this biodiversity.

This report is based on the work of Pond Conservation in the UK .